This invention relates to current source inverter fed induction motor drives and more particularly to a control for extending the high frequency operation of an induction motor supplied by current source inverters.
One important feature of a current source inverter is its ability to operate satisfactorily down to zero frequency without commutation difficulties. However, current source inverters have a severely limited high frequency ability since the commutation interval (the time interval required to transfer current from one switching device to another) is much longer for this type inverter than for a voltage source inverter. The solid state devices used in current source inverters are capable of much higher switching frequencies than are utilized because of the upper frequency limits imposed by the commutation process. In general, it is very desirable to increase the output frequency as much as possible since this permits a reduction of size and cost of the associated motor.
At high frequencies, the limitations of commutation are first encountered at light loads (i.e. no load). Above a certain point, the stator frequency becomes sufficiently large that instability occurs. This condition is reached when commutation overlap (time between gating an oncoming thyristor and extinction of the previously conducting diode) becomes 120.degree.. It has been shown that an approximation to this critical frequency can be written in the form ##EQU1## where L.sub.k is the commutation inductance per phase (i.e., the sum of the motor leakage inductances,
L.sub.m is the motor magnetizing inductance per phase, PA1 and C' is the effective capacitance during commutation.
An examination of equation (1) indicates that the critical frequency can be increased by decreasing the capacitance to as small a value as possible. However, this procedure rapidly becomes impractical because of the increasingly large voltage spikes caused by the shorter commutation time which, in turn, causes increased stresses on the motor winding insulation and increases the voltage rating of the inverter diodes and thyristors. The critical frequency can also be made large by reducing L.sub.m. However, only relatively small changes in L.sub.m can be accomplished without substantially increasing the excitation losses of the machine. Also, only relatively small changes in the critical frequency can be introduced by changes in L.sub.k.
It is an object of the present invention to provide a control for a current source fed induction motor drive which greatly extends the high frequency operation capability.